Carbon-carbon composites are important materials for aerospace applications because they retain high strength and toughness at high temperature. However, the use of carbon-carbon composite material is presently limited because of its susceptibility to oxidation. Oxidation protection may be provided by depositing a protective layer on the surface of the carbon-carbon composite which is resistant to oxidation. A preferred material is boron nitride.
Heretofore all attempts to form a boron nitride coating by chemical vapor deposition resulted in a coating having a thermal expansion substantially higher than the thermal expansion of the carbon-carbon composite substrate. A conventionally deposited pyrolytic boron nitride coating has a thermal expansion on the order of three times the thermal expansion of a typical carbon-carbon composite material. The mismatch in thermal expansion causes cracks to form in the boron nitride coating when cycled from deposition temperature to room temperature.
The method of the present invention produces a "crack-free" pyrolytic boron nitride coating over a carbon-carbon composite structure to provide oxidation protection for the composite structure at temperatures of up to 800.degree. C. and higher. For purposes of the present invention "crack-free" means a coating essentially free of cross-plane cracks which would allow air to penetrate to the carbon-carbon composite. The presence of cracks parallel to the deposition surface are not as serious provided they are not accompanied by such cross-plane cracks. Accordingly, as long as there are essentially no cracks which penetrate in a direction transverse to the deposition surface, the coating is considered "crack-free".